An EGR device that lowers combustion temperature of a diesel engine to reduce NOx generation has been known.
A part of the exhaust gas can be securely returned to supply side when exhaust-side pressure is higher than air-supply-side pressure in such EGR device. However, when, for instance, the engine is driven at medium-high speed and high-load range, the pressure of air supercharged by an exhaust turbocharger and the like may become significantly higher than exhaust gas pressure in accordance with drive condition of an engine. Even when a venturi is provided to an EGR line, the exhaust gas does not return to the air-supply side if an EGR valve is opened under the above condition and, in addition, the air supply may flow to the exhaust side, so that sufficient air supply may not be fed to the engine.
In view of the above, it has been proposed to bypass an upstream section of air-supply line relative to the junction of EGR line with a bypass line to be connected to the exhaust side (see Patent Document 1, for instance). By opening a bypass valve provided in the bypass line when the air-supply pressure becomes higher than the exhaust pressure, a part of air supply can be delivered to the exhaust side to increase the exhaust pressure, so that the exhaust gas can be securely returned to the air-supply side through the EGR line.
[Patent Document 1]JP2001-165000 A